Lithium-iodine cells and method for making same

ABSTRACT

A lithium iodine primary cell is provided that comprises a lithium anode encasing member having an aperture therethrough. Positioned within the anode encasing member is a cathode which consists essentially of a mixture of organic charge-transfer complex and iodine. The cathode includes a current collector having a lead portion with an insulating coating positioned through the aperture in the encasing member. Optionally, the current collector may include an insulating frame around its periphery to prevent electrical contact with the anode. A lithium iodide electrolyte is coextensively positioned between and in contact with the inner surface of the encasing member and the cathode. 
     The method for making the cell includes forming a receiving vessel of lithium and positioning the cathode current collector therein. The cathode material is heated to a flowable consistency and the vessel is completely filled with the heated cathode material. The completely filled vessel is chilled to a temperature sufficient to solidify the heated material; and, while in the solidified state, the vessel is sealed by positioning a lithium cap on the solidified material and cold welding the cap under pressure to the receiving vessel to form the lithium anode encasing member. Second and third redundant encasements are effected by placing a thin layer of iodine-resistant cement and an outer layer of iodine resistant film (preferably a fluoropolymer), respectively, over the receiving vessel. These encasements provide additional electrical insulation from a hermetically sealed outer metal case finally positioned over the assembly.

This is a division of application Ser. No. 564,755 filed Apr. 3, 1975.

FIELD OF THE INVENTION

The present invention relates to lithium-iodine primary cells and amethod for making same, and, in particular, to a primary cell having alithium anode encasing member for containment of the cathode andelectrolyte which is coextensive with the inner surface of the encasingmember.

BACKGROUND OF THE INVENTION

Primary cells having charge transfer complexes such as iodine-containingmaterial are known. For example, cells utilizing iodine-containingcharge transfer complexes as cathodes and anodes of selected divalentmetals have been disclosed by Gutman et al, J. Electrochem. Soc. 114,323 (1967). Also, high energy density batteries utilizing a lithiumanode and cathode of organic materials such as polycyclic aromaticcompounds, organic polymers, heterocyclic nitrogen containing compoundsand the like and iodine have been disclosed, U.S. Pat. No. 3,660,163.

Improved cathode compositions comprising a mixture of iodine andpoly-2-vinypyridine .sup.. nI₂ or poly-2-vinyquinoline .sup.. nI₂, wheren = 2-15, have been taught. See U.S. Pat. No. 3,674,562, incorporatedherein by reference. Cathode material of the latter type is typically apliable, plastic-like solid. In other applications the cathode materialis viscous substance that has a proclivity to flow.

Since the lithium halide batteries are typically used with implantableprosthetics such as cardiac pacemakers, it is necessary that they bephysically small, and highly reliable. Various lithium batteryenclosures have been proposed to contain the iodine cathode materialfrom flowing and forming a short circuit between the anode and cathode.For example, U.S. Pat. No. 3,723,183 a lithium battery enclosure isdisclosed in which a lithium anode is formed as the inner surface of asteel battery container. The iodine-containing cathode material is thenpressed into the lithium layer. A shoulder is thereafter formed on thelithium layer and precision fit with a cap to contain the cathode. Thecap includes an aperture through which an insulated lead provideselectrical contact with the cathode. A material such as epoxy is formedover the cap to hermetically seal the battery.

Enclosures such as the one described in U.S. Pat. No. 3,723,183 have anumber of commercial disadvantages. For example, the voided spaceprovided between the cathode material and cap physically enlarges thesize of the battery as well as permits oxidation of the lithiummaterial, even with dry air. Further, it is difficult in practice tomaintain the cap receiving shoulder sufficiently clean to obtain adiffusion bond or weld when the cap is pressed thereon. Thus, there isno way to check whether the battery will leak until it has beencompletely assembled with the hermetic seal. A further disadvantage isthat the outer case is necessarily the negative terminal of the cellsince it is common with the lithium.

One commercially satisfactory method of assembling lithium-iodine cellsutilizes an internal plastic encapsulating member to contain the activecell components. In these batteries, a lithium anode surrounded by thecathode material is encased in a plastic case which is fitted within ametal outer case. The plastic encasements, however, have two substantialdisadvantages: (1) the plastic encasing material occupies space whichcould be preferentially used for the active cell material, and (2) theadhesive used to cement the plastic material closed is subject to attackby the active species from the electrolyte or cathode causing physicaldeterioration and/or a decrease in resistivity.

Accordingly, it is an object of the present invention to provide alithium anode cell that minimizes the quantity of plastic to affordeither a size reduction in the completed assembly or utilize more activecomponents. It is a further object of the invention to provide a cellthat has no air or void spaces, shows increased contact area betweenanode and electrolyte, and can be tested for leakage prior to completeassembly. Further, it is an object of the invention to provide a cellhaving increased voltage under load, decreased internal impedance, and alarger electrical capacity compared to plastic encased lithium anodecells of the same size.

SUMMARY OF THE INVENTION

The present invention provides a lithium-iodine primary cell thatovercomes the disadvantages and inherent limitations of prior art cellsand which is particularly well suited for use in prosthetic devices suchas cardiac pacemakers. Generally, the primary cell of the presentinvention comprises a lithium anode encasing member having an aperturetherethrough. A cathode is positioned within the anode encasing memberand consists essentially of an organic charge transfer complex andiodine. The cathode preferably includes a current collector having alead portion with an insulating coating thereon that is positionedthrough the aperture. The aperture may comprise a slit in the lithiumor, alternatively, may be formed by enveloping the insulated cathodelead between folds in the lithium used in assembling the receivingvessel. In both cases, the lithium is bonded to the plastic insulatorunder pressure to form a seal. A lithium iodide electrolyte, preferablyformed in situ, is coextensively positioned between and in contact withthe inner surface of the anode encasing member and the cathode.

In a preferred embodiment, the lithium anode encasing member includes ananode current collector having a lead portion. The encasing member isprovided with a layer of iodine-resistant cement and a thin plasticinsulating coating, and the coated assembly is hermetically sealed in ametal protective case made, preferably, of stainless steel.

The cathode material is preferably a charge-transfer complex of organicmaterial and iodine. Charge-transfer complexes are a well-known class ofmaterials that have two components, one an electron donor, and the otheran electron acceptor, that form weakly bonded complexes that exhibitelectronic conductivity higher than either component. Thecharge-transfer complexes are in chemical equilibrium with small amountsof free iodine that is available for electro-chemical reaction. Cathodescontaining intimate mixtures of low-conductivity complexes with powderedgraphite or inert metal have high conductivities and can provideperformance comparable to cells using high-conductivity complexes.Suitable charge complexes may be prepared using an organic donorcomponent such as polycyclic-aromatic compounds, e.g., pyrene,anthracene, and the like; organic polymers, for example, polyethylene,polypropylene, polyvinyls; or heterocyclic compounds containing nitrogenor sulphur, e.g., phenothiazine, phenazine, and the like. Preferably,the charge transfer complexes comprise a mixture of iodine and solidpoly-2-vinyl pyridine .sup.. I₂ or poly-2-vinylquinoline .sup.. I₂.

The electrolyte is preferably lithium iodide which may be formed in situby contacting the anode and cathode surfaces wherein the lithium reactswith iodine in the cathode to form a solid lithium iodide electrolytelayer that contacts both the anode and the cathode. Alternatively, theelectrolyte includes a coating of lithium iodide or lithium halide onthe lithium anode formed by reaction of the lithium with iodine or otherhalogen.

In a preferred embodiment of the invention, the lithium-iodine cell ismade by forming a sheet of lithium metal into a receiving vessel havingan aperture therethrough and an opening. The vessel is formed to includeat least one extending portion having a shape that conformsdimensionally to the opening and which is adapted to sealingly close theopening. A chemically resistant material, for example, expandedzirconium metal or rigid foil, is welded to a plastic-coated lead wireto form a cathode current collector. The cathode current collector ispositioned, preferably, in the center of the formed vessel such thatcoated lead wire is securely located through the aperture. Alternativemethods of accurately positioning the cathode lead comprise positioningthe coated lead between the folds of lithium during the pressure formingof the lithium receiving vessel and/or heat sealing a plastic framearound the periphery of the cathode current collector where the frame isdimensioned to precisely fit the interior of the receiving vessel.

The cathode, preferably an organic charge transfer complex and iodine,is heated to a flowable consistency, for example, to between 200° and225° F. The formed vessel is then completely filled with the heatedcathode material to provide intimate contact with all of the innersurfaces of the lithium vessel and cathode current collector. The vesselis chilled, for example, to a temperature to between -130° and -65° E,to solidify the cathode material. While the cathode material is in thesolidified state, the extending portion of the vessel is positioned inabutting relationship with the cathode material and the peripherialedges of the opening are pressed against the vessel and cold-welded toform the anode encasing member.

Other advantages of the present invention will become apparent from aperusal of the following detailed description of a presently preferredembodiment taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional elevation of a lithium-iodine battery of thepresent invention;

FIG. 2 is a plan view of the cut and fold lines of a sheet of lithiumused to form the lithium anode encasing member;

FIG. 3 is an isometric view of the lithium sheet of FIG. 2 formed as areceiving vessel;

FIG. 4 is a partial elevation in section of an alternative cathodecurrent collector for use in the battery shown in FIG. 1;

FIGS. 5 and 5a are plan views of a cut sheet of lithium used in analternative embodiment of the invention;

FIG. 6 is a plan view of a sheet of lithium of FIG. 5 positioned in adie used to press the folds into a receiving vessel; and

FIGS. 7 and 7a are front and side elevations of a cathode lead tool usedin connection with the die shown in FIG. 6.

PRESENTLY PREFERRED EMBODIMENT

Referring to FIG. 1, lithium-iodine cell 10 of the present invention isshown as hermetically enclosed within metal casing member 11, preferablyof stainless steel. Cell 10 includes a lithium anode encasing member 12having an aperture 13 at the top thereof. Cathode 14 comprising aniodine-containing organic charge-transfer complex, preferably, a mixtureof iodine and solid poly-2-vinylpyridine .sup.. nI₂ orpoly-2-vinylquinoline .sup.. nI₂ (where n = 3-15), is contained withinlithium anode encasing member 12.

A lithium-iodide electrolyte 16 is coextensively positioned between andin intimate contact with the inner surface of lithium anode encasingmember 12 and the outer surface cathode material 14. The lithium-iodideelectrolyte 16 is formed spontaneously when the cathode material ofcathode 14 is brought into contact with the inner surface of the lithiumencasing member 12.

Cathode 14 includes cathode current collector 17. Cathode currentcollector 17 preferably comprises a metal screen 18, for example ofexpanded zirconium metal, spot welded to a metal foil 19, preferably ofzirconium. Cathode current collector 17 includes a lead wire 21 having aplastic insulating coating 22 thereon. Lead 21 is positioned in aperture13 and extends through glass-metal seal 23 in the top portion 32 ofmetal casing member 11 for external connection.

Lithium anode encasing member 12 includes anode current collector 24preferably comprising a screen-like material, for example, zirconiumscreen 26, and a lithium foil 27 positioned thereover and press bondedto the outer surface to encasing member 12 to provide alithium-to-lithium bond. Anode current collector 24 includes a lead wire28 which is directed through glass-metal seal 29 to provide an externalcircuit connection.

Lithium anode encasing member 12, including anode current collector 24,is provided with a coating 30 of iodine-resistant cement, such asalpha-cyanoacrylate, to provide electrical insulation and to bondthereto a thin layer of insulating material 31 such as TFE Teflon, Halaror FEP Teflon. Coating 31 is typically 0.0005 to 0.005 inches thick andis adapted to provide both electrical insulation between encasementmember 12 and metal case 11, and a redundant encasement of the cathodeor depolarizer material 14. Metal casing 11 includes a top portion 32which is welded to the top of case 11 to provide a hermetic seal afterreceiving the lithium anode encasing member 12.

With reference to FIGS. 2 and 3, a preferred method is shown anddescribed for making the primary cell of the present invention. Whilethe method discloses a cell of rectangular configuration, it is clearthat the method may be modified in accordance with the teachings hereofto form a cell having any desired configuration, for example, acylindrical cell. Referring to FIG. 2, in particular, a sheet of lithiumfoil 11' having a thickness of preferably 0.040 inches is cut into arectangular pattern having a width of about 3.625 inches and a length ofabout 2.15 inches. The pattern includes two extending portions, A and B,having lengths of 3.158 inches and 1.393 inches, respectively, and awidth of 0.388 inches.

Fold lines are preferably inscribed on lithium foil 11' to facilitateshaping the pattern into a receiving vessel. Metal foil 11', therefore,includes a first pair of fold lines 41 and 42 which are adapted to befolded to comprise the side walls of the receiving vessel. A second pairof fold lines 43 and 44 are provided to define flaps C and D, which areadapted to close the opening in the formed receiving vessel. These flapspreferably have a width equal to 0.388 inches. A third pair of foldlines 46 and 47 are provided which define end E. Centrally positioned inend E is aperture 13 extending therethrough and adapted to receivecathode lead 21. Four cuts 48 are provided along fold lines 46 and 47,each extending from the peripheral edge of foil 11' to the respectivefold lines 41 and 42. Corner portions 49 defined by fold lines 41-44 areremoved from foil 11' to dimensionally conform flaps C and D to theopening in the subsequently formed receiving vessel.

Prior to forming the receiving vessel, anode current collector 24 isfabricated on the outer surface of foil 11'. Anode collector 24 is made,for example, by using a zirconium expanded metal screen 26 approximately0.005 inches thick by 1.50 by 1.00 inches and positioning said screen 26on the outer surface of foil 11' as shown in FIG. 3. A rectangular pieceof lithium foil 27, approximately 0.010 inches thick, is placed on topof screen 26 and pressed thereinto, bonding the lithium to lithium andembedding the zirconium screen into foil 11'. Preferably, wire lead 28is welded to the screen prior to assembly.

Cathode current collector lead 21 having coating 22 is inserted inaperture 13. Lithium foil 11' is then folded along respective fold lines41, 42, 46 and 47 to form receiving vessel 50, as shown in FIG. 3 andpressed at 5,000 pounds in die to cold weld the flaps together. Duringthe pressing operation, coating 22 is bonded to lithium and a cementsuch as cyanoacrylate is preferably used at the point lead 21 passesthrough aperture 13 to insure maximum reliability.

To provide increased stability, cathode current collector 17 ispreferably made of an expanded zirconium metal screen 18, having athickness of approximately 0.005 inches and a dimension of 1.50 inchesby 1.00 inches to which is spot welded a zirconium foil 19 having thesame dimensions but of a thickness of 0.001 inches. A foil having athickness of about 0.004 inches may be used as the current collectorwithout screen 18. Lead 21 having fluoroplastic coating 22 is spotwelded to the screen/foil current collector 17. Preferably, lead 21 isaffixed to the periphery of aperture 13 by a plastic/metal cold weldachieved in the pressing operation of receiving vessel 50. The cementingof aperture 13 as set forth above provides a redundant seal.

With reference to FIG. 4, current collector 17 preferably includes afluoroplastic frame 20 heat sealed around the periphery of thescreen/foil assembly. Frame 20 positions current collector 17 inreceiving vessel 50 such that the outer peripheal surface of the frameis actually affixed to the inner surface of vessel 50. Lead 21 having acoating 22 extending from point 73 to 74 may be positioned between thefolds of the lithium such that during the pressing operation the coatedcathode lead bonds to the lithium between the folds as shown in FIG. 4.By so positioning the coated lead, any possible leakage path ismaximized.

The cathode material is heated to a temperature of between 200° F. and225° F. to provide a flowable consistency. Receiving vessel 50 (FIG. 3)is then completely filled with the heated cathode material and theelectrolyte is formed in situ. The vessel is chilled, for example tobetween -130° and -65° F., to solidify the cathode material. Flaps C andD are folded and pressed against the solid cathode to thereby encloseit. A similar folding of flaps A and B is carried out. The cell is thenplaced in a chilled die and the top pressed at about 600 pounds toprovide a cold weld between the flaps and first encasement. A layer ofcement, preferably alpha-cyanoacrylate, is applied to the surfaces ofthe folded flaps and preferably over the entire outer surface of vessel50 to provide a second encasement for the cathode material.

The completed cell is then encased in a Halar, TFE Teflon or FTP Teflonfilm approximately 0.005 to 0.0005 inches thick. Preferably, analpha-cyanoacrylate cement is used to seal the film to the lithium andprovide a third barrier against cathode or depolarizer leakage. The cellis then fitted into stainless steel case member 11. A hermetic seal ismade by welding top portion 32 to case 11 utilizing glass-metal seals 23and 29 to seal leads 21 and 28, respectively.

With reference to FIGS. 5 through 7, an alternative method is shown anddescribed for assembling the primary cell of the present invention.Referring to FIG. 5, a sheet of lithium foil 61 having thickness ofapproximately 0.040 inches is cut into a substantially rectangular shapehaving a length of about 5.125 inches and width of about 2 inches. Anextending portion 61a having a length of about 2 inches and width of 0.5inches is provided. A second lithium foil member 62 is formed as shownin FIG. 5a having a thickness of 0.040 inches, a length of 1.843 inchesand width of about 0.437 inches. Second member 62 comprises acompression plate for the top of a formed receiving vessel.

Lithium sheet 61 is cleaned of any oxides and positioned in mold 63having a depression 64 pressed with a die, not shown, to form a pocketthat conforms to depression 64 and to provide flaps X, Y and Z as shownin FIG. 6. Positioned in the pocket formed in sheet 61 is a Tefloncathode tool 66, FIGS. 7 and 7a, and flap X is folded thereover.Positioned in slot 68 of tool 66 is a current collector, such as currentcollector 17 of FIG. 4, with an attached coated lead 21 protruding. Thislead is bent over the outer surface of folded flap X. (Lead 21 withcoating 22 is shown diagrammatically positioned at 67 on flap X in FIG.6 prior to folding only to show relative positioning.) Flaps Y and Z arethen folded over lead 21 with coating 22.

After folding has been effected, the unit, including cathode tool 66, isturned over in the mold and an anode lead is positioned on the surfaceof the lithium that had been previously juxtaposed in depression 64 andcovered with a rectangular sheet of lithium approximately 0.040 inchesthick by about 1.625 inches in length and 1.0 inch in width. Theassembly is then pressed flat at 5,000 pounds of pressure to cold weldthe anode to the lithium and to form a receiving vessel.

The receiving vessel is removed from the mold and tested for theexistence of a short between the cathode lead 21 and the lithium. A thincoating of alpha-cyanoacrylate cement is provided over the outer surfaceof the lithium. Thereafter, cathode tool 66 is removed, leaving cathodecurrent collector 17 and lead 21 with coating 22 firmly positioned inthe receiving vessel. In a preferred embodiment, collector 17 haspreviously been heat sealed to fluoroplastic frame 20. This frame aidsin maintaining the position of collector 17. The vessel is then filledwith the cathode complex material.

The filled vessel is then chilled to between -130° F. and -65° F. tosolidify the cathode complex material. Cathode compression plate 62,FIG. 5a, is fitted into the top of the filled vessel and the edges ofthe vessel are folded over the plate permitting the other end of coatedlead 21 to pass through. The top is then pressed at 600 pounds pressureusing a ram, not shown, effecting a lithium-to-lithium cold weld. Theunit is then coated with a thin layer of alpha-cyanoacrylate cement.

The lithium coated cell is thereafter positioned in an outer casingmember and a header unit placed on top to encase the cell. The cathodeand anode leads are sealed to the head by means of glass-metal seals inthe same manner as described with reference to FIG. 1.

The following table provides a comparison of lithium cells of thepresent invention and prior art plastic encased lithium anode cellshaving the same outer physical dimensions. Both cells are of theformulation:

Li/LiI/poly (2-vinylpyridine) .sup.. nI₂.

    ______________________________________                                                      Lithium anode                                                                           Prior art                                                           cell of the                                                                             plastic encased                                                     invention lithium and cell                                      ______________________________________                                        Lithium area    7.42 in.sup.2                                                                             3.76 in.sup.2                                     Cathode weight  45-50 g     34-35 g                                           Open circuit voltage                                                                          2.804 volts 2.806 volts                                       Impedance at 1000 H.sub.z                                                                     15 ohms     18 Ω                                        Capacitance     3 microfarads                                                                             2 microfarads                                     Volts under 50 K Ω load                                                                 2.787       2.795                                             Volts under 1 K Ω load                                                                  2.655       2.597                                             DC resistance   51 Ω  78 Ω                                        Electrical capacity                                                                           5.5 to 6.0  4 Amp. hr                                                         Amp. hr.                                                      ______________________________________                                    

Under load, lithium cells of the invention show a decreased polarizationwith time because of the increased lithium surface area.

While presently preferred methods of forming the lithium anode encasingmember metal have been shown, it is clear that other methods, such asdrawing, may be used. Notwithstanding the method used to form thelithium encasing member, it is necessary that the cathode material beheated to a flowable consistency and then chilled to solidify thematerial. By solidification, it is possible to completely enclose thecathode material, since the solidified cathode material provides theinternal support necessary to cold weld the lithium flaps. Accordingly,while presently preferred embodiments of the invention have been shownand described in particularity, the invention may be otherwise embodiedwithin the scope of the appended claims.

What is claimed is:
 1. A method for making a lithium iodine cellcomprising:A. forming a lithium metal sheet into a receiving vesselhaving an aperture therethrough and an opening thereinto, said vesselincluding at least one extending portion having a shape conformingdimensionally to said opening for sealing closed said opening; B.forming a cathode current collector by welding a plastic coated leadwire to a metallic screen-like material; C. positioning the cathodecurrent collector within the receiving vessel and the lead wire throughsaid aperture; D. heating a cathode material to a flowable consistency;E. completely filling said receiving vessel with the heated cathodematerial and chilling said filling vessel to solidify said material; andF. while said material is solidified, positioning said extending portionin abutting relationship with the solidified material to sealing closedsaid opening under pressure to form an anode encasing member.
 2. Amethod as set forth in claim 1 including the step of forming an anodecurrent collector on said lithium metal sheet prior to forming saidreceiving vessel, said anode current collector being formed bypositioning a metallic screen-like material having a lead portion on thesurface of sheet, overlaying said screen with a lithium foil ofconforming dimension, and pressing said foil and screen into said sheetto form a lithium-lithium bond.
 3. A method as set forth in claim 2including the step of coating said anode encasing member and anodecurrent collector with an insulating material.
 4. A method as set forthin claim 3 including the steps of positioning said coated anode encasingmember in a metal case, welding an enclosing member having a pair ofopenings therethrough to said metal case, and sealing said leads of saidanode and cathode current collector in the respective openings tohermetically seal said anode encasing member within said metal case. 5.A method for making a lithium iodine cell comprising:A. forming alithium foil sheet into a receiving vessel having an opening therein byfolding the foil and cold welding the edges of the folded foil andcutting from a lithium foil a compression plate; B. forming a cathodecurrent collector from at least a metallic foil and electricallyconnecting thereto an iodine resistant insulated lead wire; C.positioning the cathode current collector within the receiving vessel;D. heating a cathode material consisting essentially of an organiccharge transfer complex and iodine to a flowable consistency; E.completely filling said receiving vessel with the heated cathodematerial, and chilling said filled vessel to solidify said material; F.while said cathode material is solidified, positioning said compressionplate over said material in the vessel and folding the edges of saidvessel thereover and permitting the insulated lead wire to pass through;and G. cold welding said folded edges to said plate under pressure, andforcing under pressure said compression plate to make intimate contactwith said solidified cathode material.
 6. A method as set forth in claim5 including the step of forming an anode current collector on saidlithium vessel by cold welding a second sheet of lithium foil over ametallic screen-like material having a lead portion attached thereto andpositioned on the outer surface of said formed vessel.